libstdc++: Add noexcept-specifier to std::reference_wrapper::operator()

This isn't required by the standard, but there's an LWG issue suggesting
to add it.

Also use __invoke_result instead of result_of, to match the spec in
recent standards.

libstdc++-v3/ChangeLog:

	* include/bits/refwrap.h (reference_wrapper::operator()): Add
	noexcept-specifier and use __invoke_result instead of result_of.
	* testsuite/20_util/reference_wrapper/invoke-noexcept.cc: New test.

libstdc++-v3/include/bits/refwrap.h

@@ -348,8 +348,9 @@ _GLIBCXX_MEM_FN_TRAITS(&& noexcept, false_type, true_type)
 
       template<typename... _Args>
 	_GLIBCXX20_CONSTEXPR
-	typename result_of<_Tp&(_Args&&...)>::type
+	typename __invoke_result<_Tp&, _Args...>::type
 	operator()(_Args&&... __args) const
+	noexcept(__is_nothrow_invocable<_Tp&, _Args...>::value)
 	{
 #if __cplusplus > 201703L
 	  if constexpr (is_object_v<type>)

libstdc++-v3/testsuite/20_util/reference_wrapper/invoke-noexcept.cc

@@ -0,0 +1,15 @@
+// { dg-do compile { target c++11 } }
+
+// C++11 20.8.3.4 reference_wrapper invocation [refwrap.invoke]
+
+#include <functional>
+
+struct F
+{
+  int operator()() noexcept(true) { return 1; }
+  int operator()() const noexcept(false) { return 2; }
+};
+
+F f;
+static_assert( noexcept(std::ref(f)()) );
+static_assert( ! noexcept(std::cref(f)()) );